Blood gas sensor amplifier and testing system

ABSTRACT

The invention includes an amplifier for processing the output signal from an in vivo sensor for the partial pressure of gas in blood. Means are provided to protect the patient from excess currents and voltages. The first amplifying stage has a floating ground and is at substantially the same potential as the sensor. The output of this stage is chopped with a field effect transistor that derives its control voltage from a transformer which is driven at high frequency and which has high impedance or low leakage at power line frequencies. The chopped amplifier output signal is passed through another transformer which closely couples high frequencies. The amplified signal is demodulated by another field effect transistor in the secondary of this transformer after which the signal is further processed in circuitry that need not be isolated from ground. Means are provided for displaying the signal in terms of partial pressure of the gas in millimeters of mercury. The system includes means for testing the integrity of the sensor before and continuously after it is implanted in the body. Means are also provided for calibrating the sensor under known conditions which are conveniently established.

United States Patent [1 Cornelius [111 3,710,778. 1 Jan. 16,1973.

[54] BLOOD. GAS SENSOR AMPLIFIER AND TESTING SYSTEM [75] Inventor:

Wis.

[73] Assignee: General Electric Company [22] Filed: March 15, 1971 21Appl. No.: 124,364

[52] US. Cl ..l28/2 G, 73/23 R, 128/2 E,

l28/2.l E, 204/195 B, 324/30 R [5 1] Int. Cl. ..A61b 5/05 [58] Field ofSearch.....l28/2 E, 2 G, 2 R, 2 L, 2.1 E; 204/195 P; 324/30 R; 73/23 RPrimary Examiner-Kyle L. Howell Attorney-Arthur V. Puccini, Frank L.Neuhauser, Oscar B. Waddell, Joseph B. Forman and Jon Carl Gealow FrankL. Cornelius, Milwaukee,

[57] ABSTRACT The invention includes an amplifier for processing theoutput signal from an in vivo sensor for the partial pressure of gas inblood. Means are provided to protect the patient from excess currentsand voltages. The first amplifying stage has a floating ground and is atsubstantially the same potential as the sensor. The output of this stageis chopped with a field effect transistor that derives its controlvoltage from a transformer which is driven at high frequency and whichhas high impedance or low leakage at power line frequencies. The choppedamplifier output signal is passed through another transformer whichclosely couples high frequencies. The amplified signal is demodulated byanother field effect transistor in the secondary of this transformerafter which the signal is further processed in circuitry that need notbe isolated from ground. Means are provided for displaying the signal interms of partial pressure of the gas in millimeters of mercury. Thesystem includes means for testing the integrity of the sensor before andcontinuously after it is implanted in the body. Means are also providedfor calibrating the sensor under known conditions which are convenientlyestablished.

12 Claims, 6 Drawing Figures MULTI B K VIBRATUR 84 25K H 77 L 85 3 /t l3)28 OMP n4 i 26 I2\ 125 1 I27 ANTILOG QQ' S CIRCUIT m 2 I9 \76 RECORDERPATENTEDJAN 16 I975 K 3. 71 0.778

INVENTOR FRANK L CORNELIUS F|G.6

ATTORNEYS BACKGROUND OF THE INVENTION The partial pressure of bloodgases such as carbon dioxide and oxygen have been measured in vitro withpotentiometric and polarographic sensors for many years. There have beenrecent advancements in the characteristics and configurations of bloodgas sensors which adapt them for in vivo use, thereby permittingmonitoring of the partial pressure of these gases as well as the pH ofblood on a continuous basis during anesthesia and other medicalprocedures. An example of a recently developed in vivo sensor for thepartial pressure of carbon dioxide is given in the copending applicationof R. A. Macur, Ser. No. 1 10957, filed on Jan. 29, 1971 which isassigned to the assignee of this application. The sensor described inthat application is exemplary of the type that can be tested,calibrated, utilized and continuously monitored for operability by theapparatus and methods constituting the present invention.

The blood carbon dioxide partial pressure sensor disclosed in theabove-cited pending patent application is characterized by a sensingelement which is about 30 mils in diameter and a few inches long. Thissensor may be disposed in a cannula which has pierced a blood vessel sothat the tip of the sensor is exposed. to flowing blood. The sensorcomprises a fine core wire which is pH sensitive and whose'distal endextends beyond the surrounding silver tube which-is chlorided and servesas a reference electrode. The tip or distal end of the sensor issurrounded by a thin membrane which encapsulates an electrolyte thatcontacts the reference and sensing electrodes. The membrane isimpermeable'to ions but permeable to carbon dioxide from the blood. Whencarbon dioxide flows in or out of the membrane there are changes in thehydrogen ion concentration. Therefore, pH of the electrolyte changes.This pH change produces varying electric signals which are signal if thesensor is defective such as would be the case if its carbon dioxidepermeable membrane had developed a pinhole.

processed and result in a display of partial pressure in terms ofmillimeters of mercury.

After sensors of this type are made and tested, they are inserted in asealed tube which is filled with an electrolyte that is isotonic withthe electrolyte of the sensor. The sensors are stored in this conditionin a gas mixture which has oxygen and carbon dioxide components atpredetermined partial pressures that are close to the pressures of thesegases which exist in the blood of the average patient. The electrolytefilled tube is permeable to the gas for which the sensor is designed,such as carbon dioxide, in which case the isotonic electrolyteequilibrates under the storage conditions at a partial pressure ofcarbon dioxide which corresponds with the partial pressure of that gasin the mixture. Thus, the sensor may be calibrated while it remains inthe tube filled with the isotonic electrolyte at a known partialpressure of carbon dioxide.

The electrolyte-filled sensor encasing tube has a conductor whichextends from the isotonic electrolyte to the exterior of the tube. Forinstance, there may be a wire extending through the tube wall whichconnects with a metal coating on the outside of the tube. This providesmeans for checking the integrity of the sensor before it is inserted inthe patient and even before it is removed from the tube. The signalprocessing ap- .The new monitor also permits the sensor to'becalibrated. at body temperature while the sensor remains in the tube inan equilibrated state which is close to the partial pressure of carbondioxide that exists in the blood of the average patient. For thispurpose, the monitor is equipped with heating sockets which aremaintained at normal body temperature and in which the tube-encasedsensormay be deposited during the calibration procedure. This permitscalibrating any individual sensor under known temperature and partialpressure conditions and permits introducing of calibrating sample beforeeachcalibrating operation but also because it did not allow calibrationof the sensor while maintaining its sterility.

- SUMMARY OF THE INVENTION Objects of this invention include providing amonitor that will facilitate calibrating and checking the electricintegrity and operability of a partial pressure sensor before the sensoris inserted in the body.

A further object of this invention is to provide a monitor whichproduces an indication if an in vivo sensor becomes inoperative duringuse. A corollary to this object is the provision of -means for limitingcurrent flow through the body to a very small magnitude in the event ofsensor failure.

Another object is to provide a monitor that will isolate a patient whohas a sensor implanted from any ex traneous power line currentsincidental to an electrical breakdown or accident with some otherequipment in the vicinity of the patient.

A further object and feature of the invention is the provision of meansfor correcting and accounting for differences in gain or output voltageversus partial pressure that inevitably occur in mass produced items ofthis type.

How the foregoing general objects and other more specific objects ofthis invention are achieved will appear from time to time throughout thecourse of the ensuing description of the invention.

Briefly stated, the invention is characterized by the use of a firststage, isolation amplifier to the input terminals of which a partialpressure sensor may be connected. The output signals of this amplifierare chopped and transformed and then demodulated before passing throughan ensuing signal processing stage. The transformer has close couplingfor'high frequencies but high impedance or low leakage for lowfrequencies in which case power line leakage currents of significantmagnitude will not pass through the amplifier and sensor to the patientor vice versa. The chopper or modulator is also driven from thesecondary 'side of a high frequency transformer which has couplingproperties similar to those on the transformer 1 just discussed, inwhich case stray power line currents cannot pass through the amplifierand sensor. The demodulated sensor signal is further processed in anamplifier which is subject to having its offset or bias controlled incorrespondence with thetemperature of the sensor. Means are alsoprovided for matching the gain of the amplifier to the gain of thesensor. Means are provided too for adjusting the offset or bias of theamplifier so that the numerical value displayed on a digital readoutagrees with the actual partial pressure of the gas that exists in thefluid-filled tube during calibration. A high frequency is constantlyapplied across the sensor and a tuned circuit. If the sensor fails, thevoltage on the tuned circuit changes and a warning signal results.

The monitor is also provided with one or more temperature controlledsockets in which the sensor may be placed and temperature equilibratedduring calibration.

An illustrative embodiment of the new blood gas partial pressure monitorwill now be described in greater detail in reference to the drawings.

. DESCRIPTION OF THE DRAWINGS I FIG. 1 is frontal perspective view of ablood gas monitoring device embodying the invention;

FIG. 2 is a schematic circuit diagram of the monitor with a blood gassensor connected;

FIG. 3 is a plan view of one type of blood gas partial pressure sensorwhich may be used with the invention;

FIG. 4 is a magnified longitudinal cross sectional view of the distalend of the blood gas partial pressure sensor depicted in the precedingfigure;

FIG. 5 shows a sensor encased in a plastic liquid tight but gaspermeable plastic tube which is filled with electrolyte; and

FIG. 6 depicts in cross section a sensor heater block which is builtinto the monitor and used to stabilize the temperature of the sensor forcalibration purposes.

DESCRIPTION OF A PREFERRED EMBODIMENT A monitor comprising a bodytemperature measuring module 10 and a blood gas partial pressuremeasuring module 11 is shown in FIG. 1. The temperature module is anadjunct to measuring blood gas pressure but forms no part of the presentinvention. The temperature module has a digital display of temperatureas indicated by the numeral 12. There is also an on-off switch 17 Thegas partial pressure measuring module 11 has some components on itsfront panel which will be men- I millimeters of mercury. The middlesection of the module 11 has a socket 20 by means of which the pa-'tient lead to the blood gas sensor may be connected. Also provided is asmall socket 21 to which the sensor and a short lead section which isattached to it may be' connected during the calibration procedure.Adjacent these sockets is an insulatingly mounted metal test terminal22. This terminal 22 is exposed so that it may be contacted by themetalized tip of the tube in which the sensor is stored before use tocheck. the electrical integrity of the sensor. If the sensor isdefective when terminal 22 is contacted, a sensor fail light 23 will goon and the sensor is not calibrated but is discarded.

The front panel of module 11 also has a switch button 24 for controllingpower to thecircuitry within the module. The glow of a lamp 25 indicateswhen switch 24 is in its on position. Also provided is a calibratingbutton 26 which is depressed when calibration of a sensor is to beundertaken. When Cal button 26 .is depressed, an indicator lamp 27 firstcomes on with diminished brightness which is maintained for apredetermined warm-up period such as about 5 minutes. After this period,the lamp goes on brighter indicating that calibration adjustments maycommence. When the time allowed for calibration expires, indicator lamp27 goes ofi and the sensor must be reequilibrated if calibration is notcomplete.

There are also a pair of sockets 28 the interiors of which aremaintained at body temperature when the apparatus is turned on. A sensorcontained in a tube filled with an electrolyte of known gaseous partialpressure components may be deposited in either of these sockets andraised to body temperature during the front panel of module 11 and isturned in an appropriate direction for making both the digital readout19 and recorder 13 conform to the known partial pressure of the gasbeing measured in the electrolyte-filled tube which encases the sensorwhen it is in a heating socket 28 as is the case during calibration.

There is also a dual or concentric knob switch 30- which is surroundedby alphabetic and numerical graduations as shown. After a sensor ismanufactured, it is tested for its intrinsic gain in terms of outputvoltage versus a gas of known partial pressure in a sample fluid andclassified from A to K depending on its particular output voltage. Theindicator mark onknob 30 is turned to a letter on the circular scalewhich surrounds it that corresponds withthe letter which is stamped onthe envelope containing the sensor. The

gain of one of the amplifier stages in the module is thereby adjusted tomatch the gain of the amplifier to the gain of the sensor. When thesensor is inserted in the blood stream, body temperature is observed onmodule 10 and knob 30 is turned to cause correspondence between bodytemperature and the temperature 1 on the scale surrounding the knob.This also affects an amplifier offset or zero change which corrects fordepartures from the standard calibrating temperature.

A typical sensor which may be tested and used with module 1 1 isdepicted in FIG. 3. The sensor is generally indicated by the referencenumeral 31. Its distal end or tip 32 has a reduced diameter theconstruction of which will 'be described to the extent necessaryhereinafter in connection with FIG. 4. The sensor has a Luer lockadapter 33 for engaging it with the open end of a cannula, not shown,which pierces a blood vessel and admits the distal end 32 of the sensorinto the blood stream. Attached to the sensor is a short length ofco-axial cable 34 which terminates in a connector 35. This connector maybe inserted temporarily in connector socket 21 of module 11 during thecalibration procedure and it may then be detached and promptly attachedto the end of the longer patient cable which terminates in a suitablemating connector, not shown.

After manufacture and testing, the carbon dioxide sensor shown in FIG. 3is inserted in a plastic tube 36 which has moderate gas permeability andis shown in FIG. 5. This tube is filled with an electrolyte 37 which isisotonic with the electrolyte inside of membrane 48 in the distal end 32of the sensor. At an end of liquid impermeable and gas permeable tube 36is a wire 38 which is sealed in the tip of the tube and extends to ametal coating 39 which is deposited on the outside of the tip. Thisprovides a conductive path from the outside of tube 36 to the sensor 31on its inside. Preliminary to undertaking the calibrating procedure,connector 35 of the sensor is plugged into socket 21 in module 1 1 whilethe sensor remains encased in liquid-filled tube 36. The metalized tip39 of the tube is then contacted on sensor test terminal 22 of the frontof module 1 1. If the sensor is electrically defective, the sensor-faillamp 23 will light to indicate this condition in which case the sensoris discarded and another one is tested in a similar manner prior tocalibration. This precludes wasting time calibrating a defective sensoror inserting one in a patient and having to withdraw it under theextenuatin g circumstances of a surgical procedure.

The magnified cross section of the sensor tip 32 will now be brieflydiscussed in reference to FIG. 4. The tip 32 comprises a central corewire 42 which is palladium or iridium in this example and in the citedcopending application. The end region of wire 42 has a coating 43 of theoxide of the metal which comprises core wire 42. This oxide-coated wireconstitutes a pH sensitive half cell. Concentric with core wire 42 is asilver tube 44 which is spaced from the wire by an insulating layer'45.In the region 46 silver tube 44 is coated with silver chloride. Thesilver tube and chloride thus constitute a reference half cell. Afterthe components described are dipped into a sodium chloride and sodiumbicarbonate electrolyte solution which adheres and forms a film 47, amembrane 48 is formed over the film and the rest of the sensor extendingback to under adapter 33, by dipping the wetted tip into a polymer whichis dissolved in a volatile solvent. The solvent evaporates and forms acarbon dioxide permeable, ion impermeable membrane 48. The outer surfaceof this membrane is exposed to the blood and transmits carbon dioxide toand from electrolyte 47. The carbon dioxide reacts with the water in theelectrolyte to form bicarbonate ions and hydrogen ions the latter ofwhich change the pH of the electrolyte in correspondence with thepartial pressure of carbon dioxide of the blood. The potential betweenthe reference and sensing electrodes just described varieslogarithmically with the partial pressure of carbon dioxide and it isthe resulting current that produces a potential which is measured andfinally displayed as a linear analog of partial pressure by means ofmodule 11 and recorder 13.

The most common mode of failure of the sensor is for membrane 48 torupture in which case the conductivity of the sensor would increasesince the membrane is a good insulator. Moreover, the sensor willproduce erroneous signals when the membrane is ruptured. Hence, duringthe period between manufacture and calibration for use, the sensor iskept in fluid-filled tube 36 to protect the membrane, to maintain itssterility, to assure that the electrolyte will not dehydrate and toprovide a means for gas equilibrating and pretesting the sensor beforecalibration. The tube-encased sensor is also held in a sterile gaspermeable envelope, not shown, a number of which are stored on the userspremises in a gas ambient of known carbon dioxide partial pressure. Thisresults in the electrolyte within tube 36 becoming gas equilibrated withthe ambient and permits calibrating the sensor with a captured sample ofa standardized fluid.

The electric circuitry of the blood gas monitor will now be described ingreater detail in reference to the schematic diagram of FIG. 2. In' thisfigure, sensor 31 is shown connected by means of coaxial cable 34 to theterminals of either socket 21 or 20 which appears on the front panel ofmodule 11. The sensor is thereby connected to the input terminal of anisolation amplifier 51 which, in this case, has a voltage gain of about5. Maximum sensor current is about 10" amps but more usually is about 3X 10* amps. Therefore, amplifier 51 should have a high input impedancesuch as about l 0 ohms. The amplifier feedback circuitry is merelysymbolized by the conductor 52 as this circuitry can be designed by askilled electronics designer. Amplifier 51 has a floating ground and itspower terminals 53 and 54 are supplied from a voltage regulator 55 whichalso has a floating ground as will be explained in greater detail later.All floating ground points are symbolized by a small triangle in thedrawings to distinguish them from true ground. The unidirectional outputcurrent signals from sensor 31 produce on the input ofamplifier 51voltage signals that are proportional to the logarithm of the partialgas pressure sensed by sensor 31.

The output signals from isolation amplifier 51 appear on a filtercapacitor 56 and terminal 57. These unidirectional output signals areapplied to a field effect transistor 58 which acts as a chopper.Potential for driving thischopper is applied to its gate terminal 59 bymeans of a conductor 60 which in this example is connected to a 25 KHZdriving source that will be discussed i in more detail a little later.

The chopped signals from transistor 58 are delivered to the primarywinding 61 of a transformer 62. This transformer closely couples 25 KHzsignals to its secon-' dary winding 63 but exhibits loose coupling to 60Hz power line frequencies. Thus, voltages applied to primary winding 61at power line frequencies will not be coupled into the secondary winding63 nor 'will they find a current path to power line ground.

Shunted across primary winding 61 and secondary winding 63 are pairs ofoppositely poled zener diodes 64 and 65, respectively, which hold thevoltage across the windings within specified limits. Also connectedbetween the primary and secondary windings are several neon lamps 66which will serve as a bypass in the event of an excessive voltagebetween the primary and secondary windings of transformer 62. These neonlamps are installed as a protection against transient voltages which mayoccur when a patient has undergone cardiac arrest and is subjected tohigh voltage DC defibrillation; however, they will not conduct whenpower line voltages up to 240 volts AC appear across transformer 62.

Connected in a line from secondary winding 63 is another field effecttransistor 67 which acts as a demodulator for the transformed choppedsignals. The gate 68 of transistor 67 is connected by means of aconductor 69 to a high frequency source which drives the demodulatingtransistor as will be explained more fully later. A filter circuitcomprising a resistor 70 in parallel with a capacitor 71 is connected tothe drain terminal of field efiect transistor 67. The demodulatedunidirectional signal corresponds with the logarithmically varyingsignals which are derived from sensor 31 and applied to the inputterminals of isolation amplifier v 51, and this demodulated signalappears onterminal 72. Discussion of how the signals appearing onterminal 72 are further processed will be deferred for the moment untilvoltage regulator 55 and its associated cirpower fromterminals 8789which arefloating and uncuitry and a sensor condition monitoring andfailure responsive circuit are described.

The voltage regulator and its associated field effect transistor drivingcircuits will now be described. Attention is invited to a high frequencyoscillator such as multivibrator 76 which is supplied with power to itsterminals 77 from a source, not shown, that is not necessarily isolatedfrom the power mains. Multivibrator 76 is indicated as oscillating at astable frequency of 25 KHz although other relatively high frequenciesmight be adopted in other designs. The output voltage from multivibrator76 is applied to the primary winding 78 of a transformer 79 which has asecondary winding 80. Transformer 79 effectively couples high frequencyvoltages on its primary with its secondary winding but exhibits highleakage to voltages at power line frequencies. A transformer is selectedwhich will block 60 Hz to such extent that if voltages at suchfrequencies are applied to the primary winding 78 currents of less thana microampere would be produced in secondary winding 80.

The isolated secondary winding 80 of transformer 79 connects to a fullwave rectifier 81 which has the high frequency alternating voltageappearing on its terminals 82 and 83 with respect to ground 84 and DCvoltage appearing on its terminals 85 and 86. The DC voltage fromterminals 85 and 86 is applied to the input terminals of voltageregulator 55 which includesconventional components for filtering andvoltage variation sensing and correction. The output voltage fromregulator 55 appears on its output terminals 87, 88 and its floatingground midpoint terminal 89. it should be evident that there is anexceedingly low probability of power line frequencies appearing onvoltage regulator terminals 8789 by virtue of the regulator beingisogrounded.

As indicated earlier, demodulator transistor 67 has its gate 68connected to the primary winding 78 of transformer 79. Primary winding78 is not necessarily floating and field effect transistor 67 need notbe since the latter is on the secondary side of transformer 62 whichisolates transistor 67 and its associated circuitry from the circuitwhich includes sensor 31 and the body in which it is implanted. Choppertransistor 58, on the other hand, is on the primary side of transformer62 so its gate 59 is connected by means of a conductor 60 to terminal 82on the secondary side of high frequency transformer 79. As indicated,the secondary side .of transformer 79 is isolated and; hence, so istransistor 58 isolated and floating.

Means for monitoring the electric integrity of sensor 31 before andduring use in a patient will now be described. One of the terminals fromsensor 31 connects to a tuned circuit 92 by means of a conductor 93.

The tuned circuit includes an inductor 94 and a capacitor 95 which areconnected in parallel. The circuit, in this example, is tuned to 1,000Hz. Any device that presents high impedance to a-c and a low impedanceto DC may be substituted for the tuned circuit. AC is used for testingbecause most sensors polarize when DC is applied. The tuned circuitpresents .a resistance of about 30 ohms or under to the DC derived fromthe sensor. it, of course, presents a high impedance of about 10,000ohms at the 1,000 Hz voltage. When the sensor 31 is undergoing itsinitial electric integrity test and during the time, that the sensor' isin the body, a 1,000 Hz voltage is applied to the sensor which exhibitscapacitance and high dielectric strength as long as membrane 48 is.intact. This voltage is derived from an oscillator 96 whose power inputterminals 97 are supplied from voltage regulator 55. The oscillator isthus I isolated. The oscillator 96 supplies alternating current throughthe body, the sensor 31 and essentially minimum current through thetuned circuit 92 when the sensor is in use. The oscillator 96 isconnected by means of a capacitor 98 to a field effect transistor 99which has a gate terminal 100. The source and drain terminals oftransistor 99 are in series with resistor 101 which connects to sensortest terminal 22 of module 1 l and to a body contacting electrode 102through a conductor 103 which is one of the conductors in the coaxialcable leading from socket 20 of module 1 l to the pa-' tient. Electrode102 may be a type which adheres to the body asis commonly used inconnection with taking electrocardiographs. Thus a 1,000 Hzcurrent ofabout i test terminal 22 through the isotonic electrolyte '37 (FIG. 5)to sensor 31 and back to tuned circuit 92. This causes a potential ofabout 20 mv ms to be developed across tuned circuit 92 when the membrane48 of sen sor 31 is intact and has its normal good dielectricproperties.Field effect transistor 99 is normally biased to act as a low variableresistance which limits current to the body to under 3 microamperes.

If the dielectric properties of sensor 31 are degraded such as byrupture or the occurrence of a leak in membrane 48, the 1,000 l-lzcurrent will tend to increase but will be reduced and held to a lowvalue under 3 microamperes by the action of the field effect transistor99 which acts as a variable resistor. This results from applying anincreased biasing voltage to the gate 100 of transistor 99 coincidentwith a reduction of resistance through the body circuit including thesensor. There is also protection for the circuit elements in the eventof defibrillation by virtue of another neon lamp 105 being connectedbetween the 1,000 Hz source and floating ground. There is furtherprotection for the circuit elements by way of a pair of oppositelyconnected zener diodes 104 and a current limiting resistor 101 which areconnected with a terminal of transistor 99.

Potentials developed across tuned'circuit 92 are applied to anoperational amplifier 106 whose power terminals 107 and 108 are suppliedfrom the output terminals of floating voltage regulator 55. Thisamplifier has a feedback circuit which is merely symbolized by aconductor 109 since its characteristics may be devised by a skilledelectronics designer and would depend on the particular amplifier used.The amplifier 106 may have a gain of about 30, for instance. Itsamplified output signals are coupled through a capacitor 110 to acomparator amplifier 115. There is a diode 111 and a current limitingresistor 1 14 connected to the input terminal of comparator 1 15. Theoutput signals from amplifier 106 are subjected to some filtration bythe combined action of resistor 112 and a capacitor 113. It should beevident that when the output voltage signals from amplifier 106increase, due to increased potential across tuned circuit 92, a voltageincreasing toward pinch-off will be applied to gate terminal 100 ofvariable resistance type field effect transistor 99 which was mentionedin the preceding paragraph. This voltage is developed across a resistor116 which is supplied through a pair of series connected diodes 1 l7.-Thus, as the voltage from amplifier 106 increases, the gate voltage ontransistor 99 will be raised toward pinch-off so its resistance willincrease and its conductivity willbe reduced. This limits currentthrough the body to an acceptable level such as below 3.0 microamperesif the sensor breaks down during use.

Comparator 115 is for comparing the output signal of amplifier 106 witha reference voltage and for producing a warning signal when there is asignificant voltage change incidental to sensor failure. The referencevoltage source for comparator l comprises a voltage divider havingseries connected resistors 130 and 130' connected to floating ground atone end and having the other end terminal 118 connected to thepositive'terminal of floating voltage regulator 55. An intermediatepoint of the voltage. divider is connected to a current limitingresistor 1 19 to an input terminal of comparator 115. The referencevoltage developed on this input terminal is stable.

The comparator has power supply terminals 120 and 121 which are suppliedfrom floating voltage regu-' lator 55. The output circuit of comparatorl 15 includes a diode 122 whose cathode is connected to the base of atransistor 123. The collector of transistor 123 is supplied through aterminal 124. The collector-emitter circuit of transistor 123 includes arelay coil 125 which is insufficiently energized during operation'of anormal and operative sensor to close its relay switch contacts 126.However, if sensor 31 fails, the output of comparator 115 rises, therebyincreasing the conductivity of transistor 123 and causing relay coil 125to operate and close its contacts 126. Upon this event, a warning lamp127 goes on to provide a visual indication of sensor failure. Thevoltage for driving indicator lamp 127 may be applied to terminal 128from any suitable low voltage DC source.

A feedback circuit 129 associated with comparator 115 is shown in blockform. This feedback is in the nature of a relaxation oscillator or ablinker; which alternately conducts and discharges so as to apply andremove the potential from relay coil 125, thereby causing indicatinglamp 127 to blink. A failure that causes high conductivity throughsensor 31 increases the output of comparator 115 and causes indicatinglamp 127 to blink at ahigher rate.

The sensor integrity testing and checking circuit may be connectedslightly differently than shown in FIG. 2. For instance, drain terminalof transistor 99 could be connected directly to-the top of tuned circuit92 in which case the tuned circuit should be disconnected from amplifier106. The cathode of top zener diode 104 of body contacting electrodecircuit 103 should then be connected to the input of amplifier 106 and afixed circuit element such as a load resistor, capacitor or atunedcircuit may be connected to floating ground from the amplifier 106input. The voltage developed across the fixed element will then berepresentative of sensor impedance. The voltage developed acrossterminals 20 and 21 with suitable load 40 will represent impedancechanges due to body temperature varia-' tions. This voltage may beprocessed to produce a temperature compensating signal or it may be usedin other ways.

A description of the sensor signal processing system will now beresumed. The unidirectional, demodulated, logarithmically varying sensorsignals are conducted from demodulator transistor 67 and terminal 72over a wire 131 to an amplifier 132. The power to terminals 133 and 134of amplifier132 may be supplied from any suitable DC source since thispart of the circuitry is not floating. Amplifier 132 is involved incalibrating the sensor and in producing output signals which areconverted to a readable form.

Amplifier 132 has several inputs to which various circuits are connectedfor adjusting the gain and bias of the amplifier. Adjustment forinherent sensor gain will now be considered. Connected between an outputterminal 136 from amplifier 132 and an input terminal 137 is a feedbackcircuit which includes a resistor 138 shunted by a capacitor 139. Theseelements are in series with a wiper arm 140 which may bemoved byoperating control knob 30 on module 11 for selective contact with any ofthe stationary contacts A K which are connected to points intermediate atapped resistor 141. Resistor 141. in conjunction with a trimmingvaria-v ble resistor circuit 142 acts as a voltage divider which can beadjusted by moving wiper arm 140 to vary the feedback of amplifier 132and thereby set its gain at a desired level. Recall the earlierdiscussion of how the package of each sensor is marked with a letterafter manufacture to classify it in respect to its inherent gain and toproperly match it with the amplifier 132. As inamplifier 132 asdescribed.

his desirable to calibrate all sensors at a fixedtemperature such asaverage body temperature 37 C. It is necessary to set the zero ofamplifier 132 or to subtract a voltage at the input of amplifier 132 tocorrespond with this sensor temperature during calibration. Sensors ofthis type have a temperature coefficient and must be compensated atdifferent temperatures. A voltage divider is provided for establishingthe proper temperature compensating bias voltage on amplifier 132. Thedivider comprises a series of resistors 145 which are tapped atintermediate points to provide contacts which are labeled in FIG. 2 tocorrespond with temperatures ranging from 29 to 40 C. In series withresistors 145 is a trimming resistor 146 and a fixed resistor 147 whichconnects to a power terminal 148. The resistor tap marked 37 C connectsdirectly by means of conductor 156 to one input terminal of amplifier132 through a switch 149 and a summing resistor 178 during sensorcalibration so calibration always occurs at 37 C. This situation existswhen the switch is in contact with its B terminal. As will be explainedsubsequently, switch 149 is operated by a relay coil 150. Whencalibration is initiated, relay coil 150 is energized so as to transferswitch 149 to its B contact and apply a voltage representative of 37 Ctemperature to the input of amplifier 132. When calibration is complete,relay 150 is de-energized and switch 149 is transferred to its. A

position automatically, in which case the signal is applied through theA terminal and summing resistor 178.

After, or even before calibration, the wiper 151 of volt age divider 'istransferred to a tap that corresponds with the prevailing temperature ofthe patients body.

This temperature setting, is ,acco'mplishedxby turning:

with a resistor 153 which is in series with fixed resistors 154 and 157that are connected between ground and a voltage supply terminal 155.Voltage on the wiper 152 is applied to the amplifier input through asumming resistor 177. Since the fluid electrolyte 37 within plastic tube36 is equilibrated to correspond with a carbon dioxide partial pressureof 42 millimeters of mercury in this example, the bias of amplifier 132may be adjusted by means of knob 29 until this value appears on thedigital readout 19. Of course, the sensor could be equilibrated at otherpartial pressures as long as they are always consistent for calibration.

Calibration of all sensors at a uniform temperature is accomplished bydepositing a tube-encased sensor in one of the heater sockets 28 whichare shown on the front panel of modulell and arejn the heater block 158depicted in FIG. 6. This heater block has three recesses .159 161 inwhich there are individual resistive heater elements such as 162 inrecess 161. There is a thermistor temperature sensor'l63 in at least oneof the recesses such as 160. A temperature regulator 164 from which theresistive heating elements are supplied is controlled by thermistor 163.The heaters may be supplied with low voltage AC such as from the inputterminals 165 which are in circuit with the regulator contacts, notshown. When the sensor to be calibrated is in one of the sockets 28,momentary contacting calibration switch 26 is depressed on the frontpanel of module 11. This switch is also shown inFlG. 2. Momentarycontact by switch 26 energizes a time delay and triggering circuit whichis shown in block form and labeled 167 in FIG. 2. This event supplies atriggering voltage to the gate terminal 168 ofacontrolled rectifier 169which is thereby rendered conductive to start a timing interval. Whenrectifier 169 is conductive, a circuit is completed from a low voltageline terminal 170 through rectifier 169, resistor 174 and relay coil150.

This effects transfer of switch 149 to its B contact. 7

Also, when controlled rectifier 169 is conductive a circuit is completedthrough resistor 174, diode 171 and tube 36 are certain to beat atemperature of 37 C,

one of the dual knobs 30 on the front panel of module W 11. The knobeffectively controls wiper 151.

Assuming that during calibration amplifier 132 is getting a fixed biassignal from the 37 C tap of divider 145 and that the inherent gaincorrection for sensor 31 has been made by adjusting feedbatzkwithwiper140, it is then appropriate to adjust the zero, bias or ofi'set ofamplifier 132. or to subtract a voltage at the amplifier so that thepartial pressure of gas displayed by the digital readout meter 19corresponds with the partial pressure of the gas in isotonic electrolyte37 within plastic tube 36 which encases the sensor during calibration.This step of sensor calibration is accomplished by turning Cal knob 29on the front panel of module 11 so as to effect operation of a wiper 152which is shown located above amplifier 132 in FIG. 2. The'wipercooperates timer 173 acts to apply full; voltag e toiindieatorlamp:

172 which, thenJburns-at fullibrightnesswUpon' this" i event, the wiper152 of the gainfc ont'rolcireuit can be adjusted with knob 29 on thefront panel of module 10 so that the digital readout 19 is at thepredetermined value corresponding with the known partial pressure of thegas in the isotonic electrolyte. After another few minutes, timer173functions to de-energize relay coil 150,causingswitch 149 to transferto its A contact so 7 produce a readout on the digital display 19 whichcorthat amplifier 132 is corrected for whatever corresponding bodytemperature wiper 151 is set on and indicator lamp 172 goes ofi". Assoon as the sensor 31 is calibrated, it is withdrawn from sterile tube36 and inserted through a cannula so that its distal end is exposed toflowing blood in which case the sensor will responds with thepartialpressure of theblood gas which the sensor is designed to sense.

The varying output voltage from amplifier 132 is processed in an activefilter 175 from which it is delivered to a conventional antilogarithmcircuit 176 so that the ensuing digital readout device 19 will readlinearly with respect to the signals produced by sensor 31 rather thanas a logarithmic function thereof.

As indicated earlier, prior to proceeding with calibration, the sensoris tested for electric integrity and discarded if it is defective. Inreference to FIG 2, this very important step is accomplished bycontacting the metalized end 39 of fluid-filled tube 36 on a terminal 22which is similarly shown and labeled in FIG. 1. This is equivalent tocontacting the tip of sensor 31 in FIG. 2 directly onto terminal 22. Ifthe sensor is defective under this test, the potential on tuned circuit92 will change and cause failure indicator lamp 127 to glow brightly aspreviously described.

To recapitulate the calibrating procedure, the sensor is removed fromits envelope and the metalized end of the tube encasing the sensor iscontacted on terminal 22. If there is no indication of sensor defect,the tubeencased sensor is deposited in a heater socket and brought up tobody temperature. The gain of the last stage amplifier 132 is thenadjusted for inherent gain variations between sensors as determined bythe letter markings on the sensor package. The calibration switch 26 isdepressed and indicator lamp 27 goes on at partial brightness. Bias oroffset in accordance with temperature is automatically controlled. Afterminutes indicator lamp 27 goes on at full brightness. The calibrationknob 29 is then turned to set the bias of the amplifier 132 so that thepartial pressure exhibited by digital readout l9 agrees with the knownpartial pressure of the gas in the test fluid within tube 36. The tube36 is then removed and the sensor is inserted in the body at which timethe gain of the amplifier is adjusted for existing body temperature bymeans of control knob 30. The sensor may remain in the body during theseveral hours required for surgery or even for days when continuousmonitoring is required. It is not contemplated that the sensor will besterilized and reused but that it will be thrown away after use in anindividual patient. Although the new apparatus and calibrating procedurehas been described in considerable detail'with respect to use with onetype of sensor using a membrane, such description is to be construed asillustrative rather than limiting for the invention may be variouslyembodied and used and is to be limited only by construing the claimswhich follow.

I claim:

1. For use with a sensor which responds to partial pressure of a gasin-a body fluid of a patient to produce a signal representative of thepartial pressure and which has a predetermined electrical impedance inits normal condition, a monitor comprising:

a. terminal means for connection to the sensor b. sensor signalprocessing means having an input connected with said terminal means andadapted to produce a signal corresponding with partial pressure of thegas,

c. testoscillator means adapted for being connected to the body of thepatient to impose a substantially constant alternating voltage from theoscillator means on the sensor,

to develop a signal whose magnitude depends on the impedance of thesensor to the alternating voltage,

e. means responsive to signal variations from said circuit means whichcorrespond with a change in the impedance of the sensor, and

f. means which respond to said last-named means to produce a signalindicative of said impedance change.

2. The monitor set forth in claim 1 including:

a. a test terminal connected to said test oscillator means,

b. said test terminal being exposed for being co'ntacted by the sensorwhen it is not implanted in the body, to thereby impose alternatingvoltage from said test oscillator means on the sensor for checking itselectrical integrity.

. The monitor set forth in claim 2 further including:

a variable resistance device connected in circuit with said oscillatormeans and said test terminal,

the resistance of said device depending on its bias voltage, and I anamplifier responsive to the signal magnitude on said circuit means toproduce an output signal that is proportional to said signal magnitudeand is applied to said variable resistance device to change its biasvoltage and increase its resistance, whereby to limit current throughthe sensor if the sensor is in abnormal condition.

4. The monitor set forth in claim 3 wherein:

a. said variable resistance device is a field effect transistor having asource, a drain and a gate terminal, and I b. said amplifier has itsoutput signal supplied to said gate terminal to control the resistanceof said transistor.

5. The monitor set forth in claim 1 wherein:

a. said circuit means is a tuned circuit and has, a low impedance forsignals produced by the sensor in response to the partial pressure ofthe gas and has a high impedance for the signals at said test oscillatorfrequency.

6. For use with a sensor which responds to partial pressure of a gas ina body fluid of a patientto produce a signal representative of thepartial pressure and which has a predetermined electrical impedance inits normal condition, a monitor comprising:

a. first terminals for connection with the sensor,

b. amplifier means amplifying the signal produced by the sensor andhaving input terminals one of which is connected with one of said firstterminals,

. c. a device which has low impedance for unidirectional sensor signalsand higher irn pedance for alternating signals, said device being incircuit'between another of said first terminals and another inputterminal of said amplifier means,

(1. means producing a variable control signal in response'tocorresponding variations of the alternating signal on said .device,

e. an oscillator producing an alternating test signal,

ffan electrode for connection to the body of the patient,-and

connected to said electrode for applying the alternating test signal toa circuit including the sensor and said device, whereby said controlsignal minals for supplying power to said amplifier producing means willbe varied in correspondence 11. The monitor set forth in claim 9 furtherincludwith variations in the impedance of the sensor. ing: V .Themonitor set forth in claim 6 further including: a. a secondamplifiermeans having an output tera field effect transistor havingsource and drain minal and input terminals and being adapted .forterminals in said oscillator output circuit and havmultiple gain controland having one of its input ing a gate terminal that is connected toreceive 10 terminals connected to receive said demodulated said controlsignal, said control signal being effecsignal, tive to increase theresistance between said source b. a feedback circuit between said outputterminal of and drain terminals in response to an increasing alsaidsecond amplifier means and another of its ternating test signal on saiddevice corresponding input terminals, said circuit including a resistorwith decreased impedance of the sensor. that is adjustable to controlthe gain of the amplifi- The monitor set forth in claim 6 wherein: er inaccordance with the predetermined inherent said device is a parallelresonant tuned circuit gain of the sensor incidental to calibrationthereof, which produces voltage variations in conespom c. a voltagedivider having taps representative of the dence with variations in theimpedance of the sentemperature f the senso'tand means apply a sor tosaid alternating test signal. "Qk pp gpn 3 S l t d 6 0f Said taps to Themonitor set forth in claim 6 further including: said {mother of d puterm nals of said second signal chopper mean s receiving an outputsignal amphfier mFans to controlrthe amphfier 1,135 m from saidamplifier means cordance with the sensor temperature, and demodulatormeans, d. a bias calibrating voltage divider which has a tap a firsttransformer having a primary winding i i adjustab le and 'f t d h ofreceiving thelsigna'l from the chopper means and a saidmputtermmals ofsaid second amplifier means secondary winding delivering the signal tothe for mtroducmg a v t ge thereto which modifies demodulator meanswherein said chopped signal is r the Output 89 of d second amplifiermeans to converted to a demodulated signal which cor- P" P pamal'prfssfn'e responds to the partial pressure sensed by the senofethe gas ithe 81"! m much sensor sor, said primary winding having a floatingground, mused dunng cahbfanon and r a source of high frequency voltage6. a readout means dnven by the output signal from a second transformerhaving a primary winding amphfier F V connected to said source and asecondary winding The monitor Set fo'nh m clzflm 6 mfludmgi havingafloating ground, 7 35 a. a comparator ampl fier having one inputterminal voltage from the secondary winding of the second and areference voltage 1 3 w n transformer being applied to drive saidchopper aflo'ther P r al receiv ng said control means at a potentialreferenced to floating ground slgnal F output f of f comparator amph andotherwise isolated from ground by said transfier bemg the amphfied i'between the formers 40 7 reference voltage and the control signal, 10.The monitor set forth in claim 9 further includmeans operable by Saidlast'named output Signal ing: when it reaches'a predetermined magnitudedue to a. a full wave rectifier means having alternating voltan fb l theSensor; and

age supply terminals and DC terminals, said altera means fit toop'eranon f Sam nating voltage supply terminals being connected to fmeans for mdcatmg abnormal cond" the secondary winding of said secondtransformer, Y b. a voltage regulator means connected to said DC

1. For use with a sensor which responds to partial pressure of a gas ina body fluid of a patient to produce a signal representative of thepartial pressure and which has a predetermined electrical impedance inits normal condition, a monitor comprising: a. terminal means forconnection to the sensor b. sensor signal processing means having aninput connected with said terminal means and adapted to produce a signalcorresponding with partial pressure of the gas, c. test oscillator meansadapted for being connected to the body of the patient to impose asubstantially constant alternating voltage from the oscillator means onthe sensor, d. a circuit means connected to said terminal means todevelop a signal whose magnitude depends on the impedance of the sensorto the alternating voltage, e. means responsive to signal variationsfrom said circuit means which correspond with a change in the impedanceof the sensor, and f. means which respond to said last-named means toproduce a signal indicative of said impedance change.
 2. The monitor setforth in claim 1 including: a. a test terminal connected to said testoscillator means, b. said test terminal being exposed for beingcontacted by the sensor when it is not implanted in the body, to therebyimpose alternating voltage from said test oscillator means on the sensorfor checking its electrical integrity.
 3. The monitor set forth in claim2 further including: a. a variable resistance device connected incircuit with said oscillator means and said test terminal, theresistance of said device depending on its bias voltage, and b. anamplifier responsive to the signal magnitude on said circuit means toproduce an output signal that is proportional to said signal magnitudeand is applied to said variable resistance device to change its biasvoltage and increase its resistance, whereby to limit current throughthe sensor if the sensor is in abnormal condition.
 4. The monitor setforth in claim 3 wherein: a. said variable resistance device is a fieldeffect transistor having a source, a drain and a gate terminal, and b.said amplifier has its output signal supplied to said gate terminal tocontrol the resistance of said transistor.
 5. The monitor set forth inclaim 1 wherein: A. said circuit means is a tuned circuit and has a lowimpedance for signals produced by the sensor in response to the partialpressure of the gas and has a high impedance for the signals at saidtest oscillator frequency.
 6. For use with a sensor which responds topartial pressure of a gas in a body fluid of a patient to produce asignal representative of the partial pressure and which has apredetermined electrical impedance in its normal condition, a monitorcomprising: a. first terminals for connection with the sensor, b.amplifier means amplifying the signal produced by the sensor and havinginput terminals one of which is connected with one of said firstterminals, c. a device which has low impedance for unidirectional sensorsignals and higher impedance for alternating signals, said device beingin circuit between another of said first terminals and another inputterminal of said amplifier means, d. means producing a variable controlsignal in response to corresponding variations of the alternating signalon said device, e. an oscillator producing an alternating test signal,f. an electrode for connection to the body of the patient, and g. aterminal in the output circuit of said oscillator connected to saidelectrode for applying the alternating test signal to a circuitincluding the sensor and said device, whereby said control signalproducing means will be varied in correspondence with variations in theimpedance of the sensor.
 7. The monitor set forth in claim 6 furtherincluding: a. a field effect transistor having source and drainterminals in said oscillator output circuit and having a gate terminalthat is connected to receive said control signal, said control signalbeing effective to increase the resistance between said source and drainterminals in response to an increasing alternating test signal on saiddevice corresponding with decreased impedance of the sensor.
 8. Themonitor set forth in claim 6 wherein: a. said device is a parallelresonant tuned circuit which produces voltage variations incorrespondence with variations in the impedance of the sensor to saidalternating test signal.
 9. The monitor set forth in claim 6 furtherincluding: a. signal chopper means receiving an output signal from saidamplifier means, b. demodulator means, c. a first transformer having aprimary winding receiving the signal from the chopper means and asecondary winding delivering the signal to the demodulator means whereinsaid chopped signal is converted to a demodulated signal whichcorresponds to the partial pressure sensed by the sensor, said primarywinding having a floating ground, d. a source of high frequency voltage,e. a second transformer having a primary winding connected to saidsource and a secondary winding having a floating ground, f. voltage fromthe secondary winding of the second transformer being applied to drivesaid chopper means at a potential referenced to floating ground andotherwise isolated from ground by said transformers.
 10. The monitor setforth in claim 9 further including: a. a full wave rectifier meanshaving alternating voltage supply terminals and DC terminals, saidalternating voltage supply terminals being connected to the secondarywinding of said second transformer, b. a voltage regulator meansconnected to said DC terminals of said rectifier means, said regulatormeans having a floating ground and output terminals for supplying powerto said amplifier means.
 11. The monitor set forth in claim 9 furtherincluding: a. a second amplifier means having an output terminal andinput terminals and being adapted for multiple gain control and havingone of its input terminals connected to receive said demodulated signal,b. a feedback circuit between said output terminal of said secondamplifier means and another of its input terminals, said circuitincluding a resistor that is adjustable to control the gain of theamplifieR in accordance with the predetermined inherent gain of thesensor incidental to calibration thereof, c. a voltage divider havingtaps representative of the temperature of the sensor and means to applya voltage appearing on a selected one of said taps to said another ofsaid input terminals of said second amplifier means to control theamplifier bias in accordance with the sensor temperature, and d. a biascalibrating voltage divider which has a tap that is adjustable andconnected to said another of said input terminals of said secondamplifier means for introducing a voltage thereto which modifies theoutput signal of said second amplifier means to correspond with apredetermined partial pressure of the gas in the liquid in which thesensor is immersed during calibration, and e. a readout means driven bythe output signal from said second amplifier means.
 12. The monitor setforth in claim 6 including: a. a comparator amplifier having one inputterminal and a reference voltage connected thereto and another inputterminal receiving said control signal, the output signal of saidcomparator amplifier being the amplified difference between thereference voltage and the control signal, b. means operable by saidlast-named output signal when it reaches a predetermined magnitude dueto an abnormal condition of the sensor, and c. a signal means responsiveto operation of said last-named means for indicating said abnormalcondition.